One of the most popular manners in which to texture yarn is that of giving the yarn a .cent.false twist." There are a number of ways in which false twist is imparted to yarn, and one of them is by passing it over the surface of a rotating friction disc. In a typical application, a plurality of discs are used in concert with one another, the yarn being passed over the surfaces of a plurality of discs, arranged in series with one another. In some arrangements, the external surfaces of the discs contact the yarn, while in other arrangements the yarn is passed over internal surfaces.
In both systems, the yarn usually moves longitudinally in a direction parallel to the axis of rotation of the disc, but it bends around the surface of the disc, perpendicularly to tangents to the surfaces.
The false twist is imparted to the yarn by the frictional interaction between the yarn and the surface of the rotating disc. The principle is simple, but there are some major practical problems. An effective disc must have a surface with a sufficient coefficient of friction to grasp the yarn to the extent necessary to impart the false twist. Also, it must not inhibit the longitudinal movement of the yarn, for it is being passed over the surface of the disc at a high rate of speed, typically of the magnitude of greater than 200 meters per minute. Finally, it must resist wear, so that it's life expectancy is reasonable.
Many factors affect the performance of the disc, including yarn denier, tension and surface characteristics, yarn and disc speeds, and the surface characteristics of the disc. One of the most formidable problems has been coping with the fact that the linear speed of the disc surface varies as the radius, and since the yarn contacts the surface along a radius, it is exposed to different surface speeds along the line of contact. The fastest linear speed is, of course, at the largest radius of the disc; the slowest at the smallest. Interestingly, light yarns, such as nylon in the range of 15-70 denier, can easily be effectively false twisted, by the use of a disc having a surface of rubber-like material such as rubber or polyurethane. Such material grips the yarn sufficiently to impart twist, and does not inhibit the forward passage of the yarn. The wear characteristics of such a material are acceptable. However, such is not the case with heavier yarns.
When used with heavy yarns, polyurethane and rubber discs quickly fail. They grasp the yarn sufficiently to twist it, but they impede the longitudinal movement of the yarn, and the surface of the disc begins to wear. Ridges begin to form in the inner portions of the surface, which cause the yarn to vibrate, which in turn deepens the ridges. Finally, the disc fails. There are a number of theories as to why this happens, and a key one supposes that it is caused by the exposure varying linear speeds to which the yarn is subjected as it moves from the minor to the major radius on the surface of the disc.
Many attempts have been made to solve the problems associated with false twisting heavy denier yarns. Discs having surfaces with better wear characteristics have been tried, such as ceramic and ceramic types of materials, as well as metal. These were found not to impart enough twist. As a result of the inability of those working the prior art to solve the above mentioned problem, this particular mode of false twist has not been successfully used in heavy denier yarns. Thus, the process suffers commercially.